This invention relates to photovoltaic isolators, sometimes termed "optocouplers", and more specifically relates to a component which employs a dielectrically isolated LED and a novel photovoltaic stack which is edge illuminated by the LED to produce an output voltage which can be employed in any desired switching function.
A photovoltaic isolator may serve as a voltage source which can tun on a power switching device such as a power metal oxide semiconductor field effect transistor (MOSFET), as shown, for example, in U.S. Pat. No. 4,227,098, dated Oct. 7, 1980, The photovoltaic generator which is used in such photovoltaic isolators must have a relatively high output so that immediately upon the energization of the LED, a sufficiently high output will be produced from the photovoltaic stack to provide the gate power needed to drive the gates of control devices such as MOSFETs or bipolar transistors, or the like.
Photovoltaic generators are known which consist of a dielectrically isolated group of photovoltaic generators spaced over the surface of a dielectric support and electrically connected in series with one another. A photovoltaic generator of this type is shown in above-mentioned U.S. Pat No. 4,227,098. Such devices are commercially available. Dielectrically isolated, laterally spaced and series-connected photogenerator cells have the disadvantage that only a small volume, which may be only about 1 mil deep can be used for collection of generated minority carriers and low lifetime material is used. Also, the electrode system for connecting the devices in series blocks incident light. Consequently, the output current of such devices is limited. Moreover, the devices employ a relatively complex structure and are expensive to manufacture.
Photovoltaic generators have also been made of a stack of series-connected wafer elements which each have PN junctions therein arranged in the same forward conduction direction. These devices can be cut into small slabs which can be edge illuminated to produce an output voltage across terminals connected at the two ends of the stack. A device of this type is shown in U.S. Pat. No. 3,422,527, issued Jan. 21, 1969 to J. M. Gault and assigned to the assignee of the present application. Other examples of edge illuminated slabs for use as photogenerators are contained in U.S. Pat. Nos. 3,617,825, 3,653,971, 4,082,570 and 4,174,561.
An edge illuminated stack of cells is inherently superior to dielectric isolated cells since light can go as deep as desired into the slab and the carriers produced will still be collected even if they are formed, for example, 5 mils from the collecting junction. Moreover, with edge illuminated slabs, the electrical contacts between adjacent units are out of the light path.
In prior art edge illuminated arrangements used as photovoltaic generators, the output current power is limited. Thus, such devices have not been efficient enough to rapidly charge a MOSFET gate capacitance to reach a turn-on threshold voltage in a very short time. Commonly, the individual wafers of such devices employ an N type body with a shallow P type diffusion to form the collection junction. Also, relatively thick wafers have been used so that the final stack has a very large height which is difficult to illuminate evenly by a single LED located at the center of the stack.